The pass-thru disk feature of DataCore's SANsymphony-V is mainly used during migration scenarios where you want to replace existing storage systems with new ones. DataCore itself recommends using the pass-thru disk feature only for this kind of usage.

But there could be some more scenarios where you want to use the pass-thru disk feature.

First you can use this feature if you want to keep your existing storage systems but need the caching and mirroring functionality of SSY-V. This feature is even more useful if you have storage systems like HPs EVA series or any other storage system that has virtualization "on board". These systems often do not perform very well with DataCore's storage virtualization software and the way the software needs the storage systems to be configured. If this is the case, you can still keep the existing storage configuration that is optimized for your hardware setup and use pass-thru functionality to also get the benefits of storage virtualization. This even does work with pass-thru disks on both ends of the mirror. So if your existing storage systems still do perform well and are under warranty, keep them running.

Second scanrio is where you are constrained by budget (is this a real life scenario? :-) ). Buying new storage systems for both mirror sides even if you can use entry-level storage sometimes hits and impressive pressure on your budget. So you have to choose between reducing the amount of storage and spindles which has direct impact on your performance or switching to another product. Switching is not an option because DataCore is the only perfect solution for everyone out there (as far as I can see :-) ) but reducing hardware to be in budget can have a significant impact on your project. So why not keep one side on your existing storage and only replace or add the second side? Of course the performance and warranty aspects should still be kept in mind. By keeping the older hardware running you reduce needed budget and sometimes this can be an opener for such a storage project that is normally out of budget if you want to replace all in one single step.

What about DataCore's recommendation for only using the pass-thru option for migration? Hm, there is no official statement from DataCore on how long you can use pass-thru disks. There is also no statement about loosing support in such a configuration so from a customers point of view there is no reason why you shouldn't do it.

What about the technical point of view? Is it a good solution in terms of performance and stability? What about long time experience?

As far as I can tell you there is no reason either from the technical side. We do use such a configuration for one of our customers. They run a HP EVA8100 on the first side (this storage system existed before the SSY-V installation, is still under warranty and is a decent fast system) and we added some HP P2000 entry level storage systems on the other side. We use FATA and FC disks in the EVA, SAS and SATA on the P2000 to get a two tier storage model. The EVA serves nearly 60 volumes as pass-thru disks to SSY-V. All disks are mirrored in the meantime to the second side and we absolutely have a stable and quite fast HA storage solution.

This setup now runs for nearly one year without any problems. Later this year or next year when the EVA is running out of support, the customer will replace it with P2000 systems so we have an exact mirror of the hardware setup on the other side. This gives the customer the ability to spread the investments over two or three years by profiting from DataCore's features from the beginning of this project.

So to but a bottom line on this article: pass-thru disks are a very reliable way to do migrations or keep existing storage systems in use even over a long period of time.